Cellular telecommunication networks are known in which the area covered by network is divided into a plurality of cells. Each cell is provided with a base station which is arranged to communicate with mobile stations in the cell associated with the base station. The third generation standards which are currently being discussed propose to use a spread spectrum access technique and in particular the code division multiple access (CDMA) technique.
Code division multiple access is currently being used in the IS-95 standard in the USA.
With the CDMA technique currently being used and proposed, the base stations and mobile stations transmit signals over all of the available frequency range. In order to distinguish the signals, different spreading codes are applied thereto. The spreading codes are generally orthogonal. A mobile station or a base station will therefore receive a relatively large number of signals in the used frequency range. In order to isolate a particular signal, the signals are despread. The particular signal will be isolated in that the desired signal will be much stronger than the undesired signals after despreading with the appropriate code. The undesired signals will provide interference.
The capacity of a CDMA system is dependent on the level of the background interference to a desired signal. If the ratio of the desired signal to the interference does not exceed a certain threshold the quality of the service will be reduced and/or a connection relying on the desired signal may not be established or may be dropped.
In one of the third generation standards (UMTS terrestrial radio access UTRA) proposed, time division duplex (TDD) has been suggested. With TDD, base stations are arranged ,to transmit and receive at different times. The same is true for mobile stations. A slot structure is suggested.
It has therefore been recognised that it is important to keep the level of interference provided by mobile stations and the base stations to each desired signal to a minimum. Of course, each desired signal acts as interference to other mobile stations or base stations. By maintaining the level of interference to other users to a minimum, the capacity of the system can be increased and/or the quality of service improved.